Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session S27: Focus Session: Computational Nanoscience VI - Nanowires |
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Sponsoring Units: DMP DCOMP Chair: Nicola Marzari, Massachusetts Institute of Technology Room: Colorado Convention Center 301 |
Wednesday, March 7, 2007 2:30PM - 2:42PM |
S27.00001: The electrostatic and structural properties of GaN nanorods/nanowires from first-principles M.-H. Tsai, Z.-F. Jhang, J.-Y. Jiang, Y.-H. Tang, L.W. Tu The first-principles calculation has revealed that the GaN nanorod has a greatly enhanced dipole moment per area relative to that of a film, which in conjunction with the geometry effect suggests that the top surface of the GaN nanorod has a greater electrostatic attraction for gas-phase Ga and N source species than the film surface during epitaxial growth of GaN, so that nanorods grow much faster than the film. This electrostatic effect may explain the growth of nanorods protruding high above the film surface. The first-principles molecular-dynamics calculation shows that the average Ga-N bond length of the GaN nanowire decreases with the decrease of the diameter of the nanowire, which demonstrates a surface tension effect. This trend can be expected to be the same for the experimentally grown nanorods, because the physical origin that drives this contraction, namely the surface tension, is the same. Thus, the bond-length result may explain the experimentally observed blue shift of the cathodoluminescence emission. [Preview Abstract] |
Wednesday, March 7, 2007 2:42PM - 2:54PM |
S27.00002: The electronic structure of radial p-n junction silicon nanowires Shan-Haw Chiou, Jeffrey Grossman Silicon nanowires with radial p-n junctions have recently been suggested for photovoltaic applications because incident light can be absorbed along the entire length of the wire, while photogenerated carriers only need to diffuse a maximum of one radius to reach the p-n junction. If the differential of the potential is larger than the binding energy of the electron-hole pair and has a range larger than the Bohr radius of electron-hole pair, then the charge separation mechanism will be similar to traditional silicon solar cells. However, in the small-diameter limit, where quantum confinement effects are prominent, both the exciton binding energy and the potential drop will increase, and the p-n junction itself may have a dramatically different character. We present ab initio calculations based on the generalized gradient approximation (GGA) of silicon nanowires with 2-3 nm diameter in the [111] growth direction. A radial p-n junction was formed by symmetrically doping boron and phosphorous at the same vertical level along the axis of the nanowire. The competition between the slope and character of the radial electronic potential and the exciton binding energy will presented in the context of a charge separation mechanism. [Preview Abstract] |
Wednesday, March 7, 2007 2:54PM - 3:06PM |
S27.00003: Equilibrium charge and potential distribution of a surrounding-gate silicon nanowire in the LDA approximation. Bart Soree, Wim Magnus, Geoffrey Pourtois, Steven Compernolle The equilibrium charge and potential distribution of a silicon nanowire is obtained from a Poisson-Schr\"{o}dinger solver in the local density approximation (LDA). The cylindrical nanowire consists of heavily doped source and drain regions and the channel region is surrounded by a metallic gate. We have studied different cases where the low dimensionality of the wire has a profound effect on the equilibrium charge and electrostatic potential of the gated wire for different gate voltages. Our calculations show that for short channel lengths the built-in potential is significantly lowered and for small diameters volume inversion occurs. We discuss the possible implications for device performance of these low dimensional effects. [Preview Abstract] |
Wednesday, March 7, 2007 3:06PM - 3:18PM |
S27.00004: Structural and Electronic Properties of Silicon Carbide Nanowires Sanguo Shen, M. Yu, C. Leahy, C.S. Jayanthi, S.Y. Wu We have studied the structural and electronic properties of SiC nanowires (NWs) of different diameters (1 nm $<$ d $<$ 7nm) and shapes ($e.g.,$ hexagonal, round, and rhombohedral cross-sections) for wires oriented along $<$001$>$, $<$011$>$, $<$111$>$ (cut from the 3C-bulk), and $<$0001$>$ directions (cut from 2H-, 4H-, or 6H- bulk). A supercell is set-up for each of the above orientations. We relaxed the structures using the state-of-the-art semi-empirical molecular dynamics scheme as described in Ref. [1]. The main results of our findings are: (i) Among the different shapes investigated, NWs with hexagonal morphology are the most stable structures, (ii) Among the hexagonal NWs, those cut from 2H-SiC bulk structures were found to be the most stable ones in the diameter range investigated. They exhibit very weak surface relaxations, and were found to exhibit semiconductor characteristics, (iii) On the other hand, NWs cut from 3C-, 4H-, and 6H- bulk structures exhibit strong facet reconstructions and were found to have metallic characteristics. These results are in agreement with DFT-based ab-initio calculations for small diameter NWs up to 3 nms. [1] Leahy \textit{et al.} Phys. Rev. B74, 155408 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 3:18PM - 3:30PM |
S27.00005: Novel polyicosahedral Si nanowire: A molecular-dynamics study. Kengo Nishio, Tetsuya Morishita, Wataru Shinoda, Masuhiro Mikami A novel polyicosahedral nanowire, which is composed of linked icosahedral Si nanodots is spontaneously formed in a series of annealing molecular dynamics simulations of liquid Si inside a nanopore of 1.36 nm in diameter[1]. The polyicosahedral Si nanowire is stable even in a vacuum up to about 77{\%} of the melting temperature of bulk Si. Our structural energy calculations reveal that the polyicosahedral nanowire is energetically advantageous over the pentagonal one for a wire whose diameter is less than 6.02 nm, though the latter has been recently proposed as the lowest energy wire. These results suggest the possibility of the formation of a new stable polyicosahedral Si nanowire. \newline [1] J. Chem. Phys. 125, 074712 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 3:30PM - 3:42PM |
S27.00006: Investigation of surface reconstructions in [110] Ge nano-wires. Jiaxin Han, Scott Beckman, James Chelikowsky It is anticipated that nano-structures will lead to the development of novel optical devices. It is known that quantum confinement transforms indirect band-gap crystals into direct band gap nano-structures. Although is it predicted that Ge [110] nano-wires will have a direct band gap, the optical absorption spectra has yet to be measured for free standing Ge nano-wires. Recent calculations have focused on investigating the HOMO-LUMO gap in hydrogenated Ge nano-wires. (1) This surface passivation technique neglects surface states, which are suspected to be the primary recombination mechanism in these Ge nano-structures. Here we investigate the surface reconstructions of [110] Ge nano-wires using the real-space density functional theory formalism encoded in the PARSEC software. (2) The structure and electronic states associated with wires as large as 1.2 nm diameter are investigated. Simulated annealing is used to identify the minimum energy structure out of the many possible reconstructions. (1) S. P. Beckman, Jiaxin Han, and James R. Chelikowsky. Phys. Rev. B. 74, 165314 (2006). (2) http://www.ices.utexas.edu/parsec/ [Preview Abstract] |
Wednesday, March 7, 2007 3:42PM - 3:54PM |
S27.00007: {\it Ab initio} computations of structural and electronic properties of doped and undoped Ge nanowires D. Medaboina, V. Gade, S. K. R. Patil, S. V. Khare We report results of structural and electronic properties of hydrogen passivated doped and undoped Ge nanowires along [100], [110] and [111] growth directions using density functional theory in the local density approximation (LDA). Cross-sections of nanowires with diameters $\ $ d $>$ 2.0 nm are facetted reflecting the crystal symmetry about their axis. Nanowires along [100] direction with d below (above) 1.5 nm are found to be direct (indirect) band gap (E$_g$) semiconductors. Nanowires along [110] have direct E$_g$ for d $>$ 1.0 nm. Nanowires along [111] have indirect E$_g$ for d $>$ 1.0 nm. The magnitude of E$_g$ increases as the wire diameter decreases with values as high as 4.3 eV for a [100] wire with d $=$ 0.41 nm. For a fixed diameter E$_{g}^{[100]}$ $>$ E$_{g}^{[111]}$ $>$ E$_{g}^ {[110]}$. Doping with P or B did not have a significant effect on the valence and conduction band dispersions. [Preview Abstract] |
Wednesday, March 7, 2007 3:54PM - 4:06PM |
S27.00008: Electronic structures and optical properties of GaN and ZnO nanowires T. Akiyama, A.J. Freeman, K. Nakamura, T. Ito GaN and ZnO are promising semiconductor materials that exhibit many outstanding physical and chemical properties. Recently, their one-dimensional nanowires (NWs) are also attracting much interest due to their significant potential for optoelectronic nano-devices \footnote{Zhong {\it et al.}, Nano Lett. {\bf3}, 343 (2003); Ng {\it et al.}, APL {\bf 82}, 2023 (2003)}; they always take the wurtizte structure while other compound semiconductor NWs also exhibit other polytypes \footnote{Akiyama {\it et al.}, JJAP {\bf 45}, L275 (2006); PRB {\bf 73}, 235308 (2006)}. However, little is known about their electronic and optical properties. Here we investigate the electronic structures and optical properties of GaN and ZnO [0001] NWs by using the highly precise full-potential linearized augmented plane wave (FLAPW) method \footnote{Wimmer, Krakauer, Weinert, Freeman, PRB {\bf 24}, 864 (1981)}. Our calculations demonstrate that the band gap energy of both the unpassivated and passivated NWs becomes large compared with the bulk energy gap due to quantum confinement effects; surface states crucially affect the electronic structure of unpassivated NWs. Further, we find peculiar features of their dielectric functions that exhibit strong anisotropy in the calculated optical properties. Work supported by the U.S. NSF (through its MRSEC Program at NU). [Preview Abstract] |
Wednesday, March 7, 2007 4:06PM - 4:18PM |
S27.00009: DFT Study of ZnO Nanowire with Wurtzite (0001) Structure Xiao Shen, Philip B. Allen, Mark R. Pederson, Jin-Cheng Zheng, James W. Davenport, James T. Muckerman The most commonly reported ZnO nanowires have the (0001) wurtzite growth axis. We report two first-principles calculations using density-functional theory (DFT) for a small model nanowire with diameter 0.9 nm, containing 26 atoms in one periodic repeat unit cell, arranged as a fragment of a wurtzite (0001) crystal. One calculation is done on an infinite wire and the other on a truncated piece. The two calculations show excellent agreement. In both calculations, the atomic coordinates were relaxed to the nearest stable minimum. The exposed (1$\bar{1}$00) surfaces resemble closely the relaxed surface found in bulk\footnote{U. Diebold et al., Appl. Surf. Sci. 237, 336 (2004)}. Our calculation shows that the $c$ axis has a $\sim$2\% elongational strain. We also compute the change of the Young's modulus relative to the bulk, and compare our prediction with experiment\footnote{C. Q. Chen et al., Phys. Rev. Lett. 96, 075505 (2006)}. The electrical polarization of this non-centrosymmetric nanowire will be discussed, and also the assignment of a rotational quantum number $m$ to the Bloch band states. [Preview Abstract] |
Wednesday, March 7, 2007 4:18PM - 4:30PM |
S27.00010: Capillary stability of nanowires in the presence of dislocations Mark Jhon, Daryl Chrzan, Andreas Glaeser Nanometer scale structures are often unstable with respect to capillary forces. For instance, wires may be susceptible to a pearling (Rayleigh) instability or to coarsening. A simple continuum theory is presented that predicts that sufficiently small second phase wires formed around dislocations are stable to both forms of structural instability. The elastic interaction is found to balance the effects of surface energy. Infinitesimally small perturbations to an isolated wire are found to decay for wires smaller than a critical size. For an ensemble of wires smaller than the critical size, a driving force is found for inverse coarsening. These results imply that thermally stable nanometer-scale wires can be produced. [Preview Abstract] |
Wednesday, March 7, 2007 4:30PM - 4:42PM |
S27.00011: Stability of conductance oscillations in monatomic sodium wires Petr Khomyakov, Geert Brocks We study the stability of conductance oscillations in monatomic sodium wires with respect to structural variations. The geometry, the electronic structure and the electronic potential of sodium wires suspended between two sodium electrodes are obtained from self-consistent density functional theory calculations. The conductance is calculated using the Landauer-Buttiker formalism and the mode-matching technique as formulated recently in a real-space finite-difference scheme [PRB 70, 195402 (2004)]. We find a regular even-odd conductance oscillation as a function of the wire length, where wires comprising an odd number of atoms have a conductance close to the quantum unit, and even-numbered wires have a lower conductance. The conductance of odd-numbered wires is stable with respect to geometry changes in the wire or in the contacts between the wire and the electrodes; the conductance of even-numbered wires is more sensitive. Geometry changes affect the spacing and widths of the wire resonances. In the case of odd-numbered wires the transmission is on-resonance, and hardly affected by the resonance shapes, whereas for even-numbered wires the transmission is off-resonance and sensitive to the resonance shapes. Predicting the amplitude and phase of the conductance oscillation requires a first-principles calculation based upon a realistic structure of the wire and the leads. [Preview Abstract] |
Wednesday, March 7, 2007 4:42PM - 4:54PM |
S27.00012: Quantum and classical simulations of nanowire self-assembly Zhigang Wu, Jeffrey Grossman The ability to control the synthesis of nanostructures such as nanowires and nanotubes is crucial to the success of next-generation nanotechnology devices. One promising approach for efficiently controlling fabrication is to functionalize nanoscale building blocks such that they will self-assemble under the appropriate conditions. We employ a combination of ab initio total energy calculations, classical molecular dynamics (MD), and classical Monte Carlo (MC) calculations to investigate the possible self-assembly of nanoscale objects into chains and wires. The ab initio calculations provide key information regarding selective chemical functionalization for end-to-end attraction and the subtle interplay of the energy landscape, which is then used to fit classical potentials. Using these potentials, MD simulations are carried out to predict short- time (i.e., ps and ns timescales) dynamical properties of nanoparticle assembly as a function of particle shape, chemical functionalization, and temperature. Finally, both static and dynamical data from these calculations are used in MC simulations to predict large time- and length-scale assembly under a variety of synthesis conditions. Our results suggest a new technique for bringing nanoscale objects together to form ordered, ultra high- aspect ratio nanowires. [Preview Abstract] |
Wednesday, March 7, 2007 4:54PM - 5:06PM |
S27.00013: Molecular Simulation of Size-Dependent Properties of Polymeric Nanofibers Sezen Curgul, Greg Rutledge, Krystyn VanVliet Materials with nanometer dimensions have been shown experimentally to exhibit size dependent properties. Polymer nanofibers, in particular, are of interest because of their several value added applications such as medical, filtration, barrier, wipes, personal care, composite, garments, insulation, and energy storage. We report here the results of molecular dynamics (MD) simulations of polymer nanofibers using LAMMPS (Large-Scale Atomic/Molecular Massively Parallel Simulator). To date, we have simulated fibers comprised of chains that mimic the prototypical polymer polyethylene, with chain lengths ranging between C50 and C300. These nanofibers have diameters in the range 1.86-16.2 nm. The fibers have been analyzed for signature of size dependent behavior in their structural and dynamical properties. In these fibers, mass and energy density profiles are similar and they have constant bulk-like values at the center of the fiber, for sufficiently large diameter fibers. The surface layer thickness shows little dependence on the fiber size. The interfacial excess energy increases with decreasing fiber size for fibers below 5 nm in diameter. The chains at the surface show preferred conformations and orientations that are significantly different than chains at the center of the nanofiber. [Preview Abstract] |
Wednesday, March 7, 2007 5:06PM - 5:18PM |
S27.00014: Raman Antenna Effect in Semiconducting Nanowires. Gugang Chen, Qihua Xiong, Peter Eklund A novel Raman antenna effect has been observed in Raman scattering experiments recently carried out on individual GaP nanowires [1]. The Raman antenna effect is perfectly general and should appear in all semiconducting nanowires. It is characterized by an anomalous increase in the Raman cross section for scattering from LO or TO phonons when the electric field of the incident laser beam is parallel to the nanowire axis. We demonstrate that the explanation for the effect lies in the polarization dependence of the Mie scattering from the nanowire and the concomitant polarization-dependent electric field set up inside the wire. Our analysis involves calculations of the internal electric field using the discrete dipole approximation (DDA). We find that the Raman antenna effect happens only for nanowire diameters d$<\lambda $/4, where $\lambda $ is the excitation laser wavelength. Our calculations are found in good agreement with recent experimental results for scattering from individual GaP nanowires. [1] Q. Xiong, G. Chen, G. D. Mahan, P. C. Eklund, in preparation, 2006. [Preview Abstract] |
Wednesday, March 7, 2007 5:18PM - 5:30PM |
S27.00015: Structure and stability of suspended monatomic metal chains Anwar Hasmy, Luis Rincon, Raiza Hernandez, Manuel Marquez, Vladimiro Mujica, Carlos Gonzalez Since the spectacular achievement of the ultimately thin wire (a suspended monatomic gold chain) little progress has been made on the origin and the ubiquity of this phenomenon. Here we report a systematic quantum study on breaking monovalent metal nanowires through tight-binding molecular dynamics simulations. We show that at low temperature (4 K) gold, silver and copper can form linear and stable suspended monatomic chains at the late stage of the nanowire breaking process, but at room temperature silver and copper chains adopt a zigzag structure (as predicted by first principles calculations) that becomes unstable. We found that the stability and the average number of atoms forming these chains depend on the metal specie, a fact that can be explained in terms of the population of d-orbitals along the chains. Besides to clarify the controversy in the literature on the formation of these chains in 3d and 4d metals, our findings give insights on the advantages and limitations of detecting them through conductance measurements. [Preview Abstract] |
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